Tmesis

This is generally when another word is inserted in between the two segments of a compound word, usually as slang or exclamation.

Example: “un-freaking-believable”

There is a more technical definition, when the subject is inserted into a verb phrase: “turn off” becomes “turn the radio off”

Why I Looked It Up

I found it in a discussion of interactive fiction in Cybertext: Perspectives on Ergodic Literature.